Cushing's syndrome, a fatal disease, is suspected in many thousands of patients each year, but confirmed in only a fraction of these. There is no test that reliably separates individuals with pseudo-Cushing's states, who have mild hypercortisolism and minimal physical features of Cushing's syndrome, from those with mild or intermittent Cushing's syndrome, who may present in an identical fashion. We are exploiting the different pathophysiologies in the pseudo-Cushing's and Cushing's syndrome states to develop new tests for making this distinction. Since the hypercortisolism of patients with pseudo-Cushing's syndrome reflects hypersecretion of CRH, we are examining whether dexamethasone, a potent glucocorticoid, can suppress CRH production and blunt the ACTH and cortisol responses to exogenous ovine CRH in these patients. We find that a screening UFC measurement may be normal in 5% of patients with Cushing's syndrome. Midnight cortisol levels in contrast, appear to better discriminate the two groups. Studies with oCRH have revealed similar hormonal responses in Cushing's disease and pseudo-Cushing's syndrome; interestingly, dexamethasone pretreatment significantly reduces the responses in pseudo-Cushing's patients. Further work will explore whether inferior petrosal sinus sampling for ACTH and CRH levels with and without dexamethasone can distinguish between normal individuals, those with pseudo-Cushing's syndrome, and those with Cushing's syndrome. Our second initiative is to improve the approach to the differential diagnosis and treatment of the causes of Cushing's syndrome. We are characterizing the performance of the classic (standard dexamethasone suppression and metyrapone stimulation) and the newer (inferior petrosal sinus sampling, oCRH) differential diagnostic tests. Studies in 243 patients show that inferior petrosal sinus sampling has 100% sensitivity and specificity; a convenient protocol for a morning oCRH stimulation test developed in 101 patients has 100% specificity and 94% sensitivity. Re-evaluation of the standard dexamethasone suppression test has improved its specificity (100%), and retained good sensitivity (83%). The localization of ectopic-ACTH producing tumors continues to be elusive, with only 50% identified initially, despite the application of MR and CT imaging and venous sampling. This problem may yield to ongoing studies using novel ACTH secretagogues as adjuncts to venous sampling. Surgical treatment of Cushing's syndrome remains the treatment of choice, even in patients with recurrent Cushing's disease.